U.S. Pat. No. 5,983,502A discloses a shaving apparatus with a main housing and three hair-cutting units. Each hair-cutting unit comprises an external hair-cutting member with hair entry apertures and an internal hair-cutting member, which is rotationally drivable with respect to the external hair-cutting member by means of a coupling pin. The coupling pin is rotatable about a main axis and is axially movable against the force of the spring in a direction parallel to the main axis.
During shaving, a user will try to avoid skin irritation and skin injury as much as possible. Skin irritation and injury occur when the hair-cutting member comes into contact with the skin too intensively, which occurs particularly when the user presses the hair-cutting member against the skin with excessive force. This can occur during use of wet-shavers as well as electric dry-shavers, and particularly when a user switches from one system to the other or in case of an inexperienced user. In the initial period of use, it is important that the user does not press the shaving apparatus against the skin with excessive force.
The shaving apparatus disclosed in U.S. Pat. No. 5,983,502A comprises a sensor means for warning the user when a given preset force between the external hair-cutting member and the main housing is exceeded during shaving. The sensor comprises an annular permanent magnet, which is secured to the coupling pin, and a Hall-sensor, which is disposed underneath the magnet and which is secured in the main housing.
There is a direct relationship between the force with which the cutting unit is depressed (spring force) relative to the main housing and the distance between the magnet and the Hall-sensor. The distance between the magnet and the sensor decreases as the cutting unit is depressed further. The Hall-sensor is connected to an electronic circuit which has been adjusted in such a way that a warning signal is produced when a given distance, i.e. pressure, is exceeded. The shaving apparatus can produce an optical or acoustic warning signal when the pressure exceeds a given value and thus warns the user to reduce the pressure in order to preclude or stop skin irritation and injury.
The movements of the coupling pin are limited to a rotational movement about the main axis and axial movement along the main axis. Due to the annular shape of the magnet, a part of the magnet will always be located opposite the Hall-sensor, so that in each rotational position of the coupling pin and the internal hair-cutting member connected thereto, the distance between the Hall-sensor and the annular magnet can be measured.
Other movements of the hair-cutting member with respect to the main housing cannot be detected by the sensor, since it would not be possible to deduce from the measured change in distance between the Hall-sensor and the annular magnet whether the change in distance is due to an axial movement or, for example, a tilting movement about a tilt axis extending perpendicularly to the main axis.